Vigorous research is being performed on semiconductor devices that perform bipolar operations. This includes research on an IGBT in which low on-voltage is obtained by means of conductivity modulation.
Generally, in this type of semiconductor devices, there is a tradeoff such that a decrease in on-voltage of the semiconductor devices is accompanied by a decrease in breakdown-voltage of the semiconductor devices. Research is being performed to overcome this tradeoff, so that high breakdown-voltage can be maintained even when on-voltage is reduced.
One such example is set forth in Japanese Laid-Open Patent Publication No. 2001-308327. FIG. 8 schematically shows a longitudinal sectional view of essential parts of a semiconductor device set forth in Japanese Laid-Open Patent Publication No. 2001-308327. It should be noted that the configuration of the longitudinal sectional view shown in FIG. 8 is repeated in the left-right direction of the page.
In FIG. 8, a semiconductor device 300 comprises a collector electrode C, a p+ type collector region 322 connected to the collector electrode C, an n− type drift region 324 connected to the collector region 322, and a p− type body region 326 separated from the collector region 322 by the drift region 324. The semiconductor device 300 further comprises a plurality of n+ type emitter regions 328 separated from the drift region 324 by the body region 326. An emitter electrode E is connected to the emitter regions 328. A plurality of trench gate electrodes 332 is formed within the body region 326. Each trench gate electrode 332 opposes, via a gate insulating layer 334, a portion of the body region 326 that is separating the drift region 324 and the emitter region 328. Two trench gate electrodes 332 are shown on each of left-right sides in FIG. 8. The body region 326 is divided into a plurality of body sections by the plurality of trench gate electrodes 332. The body sections are classified into two groups. The body section 326a of one group has the emitter region 328 within the body section (shown on each of left-right sides in FIG. 8), and the body section 326b of the other group has no emitter region 328 within the body section (shown in a central portion of FIG. 8). A plurality of trenches 335 is formed within the body section 326b having no emitter region 328. Each trench 335 is made of a conductor 338 covered with a trench insulating layer 336. The conductors 338 of the trenches 335 are connected to the emitter electrode E. An inter layer insulating layer 339 covers a top surface of the body sections 326b having no emitter region 328. The body sections 326b having no emitter region 328 are isolated from the emitter electrode E by the inter layer insulating layer 339.
The body sections 326b having the trenches 335 are termed positive hole accumulation regions 312. The emitter regions 328 are formed repeatedly, with a determined pitch, outside the positive hole accumulation regions 312. The positive hole accumulation regions 312 are formed by locally failing to form the emitter regions 328. The positive hole accumulation regions 312 are distributed with a constant spacing within the semiconductor device.
FIG. 9 is a top-plane view of essential parts along the line IX-IX of FIG. 8. FIG. 9 shows a border between the positive hole accumulation regions 312 and a peripheral region 314. The peripheral region 314 forms a loop around a region in which switching elements are formed. The switching elements are formed from the plurality of emitter regions 328, the trench gate electrodes 332, and trenches 335, etc. The peripheral region 314 maintains breakdown-voltage by means of having a depletion layer extend from the region having switching elements towards a periphery. An FLR (Field Limiting Ring) structure or guard ring structure (not shown), or the like, is formed in the peripheral region 314.
As shown in FIG. 9, the trench gate electrodes 332 (in FIG. 9, the gate insulating layer 334 covering the trench gate electrodes 332 are shown), and the trenches 335 (in FIG. 9, the trench insulating layers 336 covering the conductors 338 are shown) extend in a y direction and are repeated in an x direction, forming a striped shape. Further, the emitter regions 328 and p+ type body contact regions 325 are formed alternately in the y direction between the trench gate electrodes 332. The emitter regions 328 and the body contact regions 325 are connected to the emitter electrode E. The semiconductor device 300 is turned-on when a voltage more positive than the emitter electrode E is applied to the collector electrode C and when a voltage more positive than the emitter electrode E is applied to the trench gate electrodes 332.
A characteristic feature of the semiconductor device 300 is that the body sections 326b within the positive hole accumulation regions 312 are not connected with the emitter electrode E. As a result, positive holes injected from the collector region 322 to the drift region 324 are prevented from passing through the body sections 326b within the positive hole accumulation regions 312 and are prevented from being discharged to the emitter electrode E. Consequently, many of the positive holes accumulate within the body sections 326b in the positive hole accumulation regions 312. The concentration of positive holes within the semiconductor device 300 thus increases, and conductivity modulation is activated. The on-voltage of the semiconductor device 300 is thus reduced. Further, the potential of the trenches 335 is fixed to the emitter potential, and consequently the effect of accumulating the positive holes is greater.
Moreover, the trenches 335 have the advantages described below when the semiconductor device 300 is turned-off. For the sake of comparison, a positive hole accumulation region 312 without the trenches 335 will be considered. In this case, an electrical field will readily be concentrated in a vicinity region 329 (see FIG. 8) of the bottom of the trench gate electrodes 332 at both ends of the positive hole accumulation region 312. This concentration of the electrical field reduces the breakdown-voltage of the semiconductor device 300. In particular, the above problem becomes serious as the width of the positive hole accumulation regions 312 increases. However, if the trenches 335 are formed in the positive hole accumulation region 312, the concentration of the electrical field is reduced, and the destruction of the semiconductor device 300 can be prevented. In particular, there is a greater need to provide the trenches 335, as the width of the positive hole accumulation regions 312 increases.